The members of this family are similar to gene products 9 (gp9) and 10 (gp10) of bacteriophage T4. Both proteins are components of the viral baseplate. Gp9 connects the long tail fibers of the virus to the baseplate and triggers tail contraction after viral attachment to a host cell. The protein is active as a trimer, with each monomer being composed of three domains. The N-terminal domain consists of an extended polypeptide chain and two alpha helices. The alpha1 helix from each of the three monomers in the trimer interacts with its counterparts to form a coiled-coil structure. The middle domain is a seven-stranded beta-sandwich that is thought to be a novel protein fold. The C-terminal domain is thought to be essential for gp9 trimerisation and is organized into an eight- stranded antiparallel beta-barrel, which was found to resemble the 'jelly roll' fold found in many viral capsid proteins. The long flexible region between the N-terminal and middle domains may be required for the function of gp9 to transmit signals from the long tail fibers. Together with gp11, gp10 initiates the assembly of wedges that then go on to associate with a hub to form the viral baseplate.